Roof Construction

by Fred on March 31, 2013

Basic Roof Trusses Design specifications for Better Construction

Approximately 75% of homes built today utilize pre-fabricated roof trusses in place of conventional roof rafters to support the roof! Roof trusses are pre-manufactured wood structures which are fabricated in a workshop and carefully designed to support the roof and the load on outside walls.

Ask most engineers today and they will confirm that engineered roof trusses are the only option and are much superior compared to the old roof frames. The major advantage of using pre-fabricated roof trusses are speedy construction and significant cost savings.

In designing roof trusses, the following aspects should be carefully considered:
# Fraction of all parts of a joint so that the maximum strength can be achieved throughout
# Provide adequate bearing surface to get the compression on the surface well within the safe limits # Spread bolts and rivets so as to provide the greatest resistance with the least cutting away of remaining structural members
# Observe that the central axis of each component coincides as much as possible with the line of action of the stress
# Examine all members and assemblies for compressive, tensile, transverse and shear forces.

It is very critical that you state correct type, measurements and other details when placing an order for roof trusses. General mistakes include incorrect specifications and less attention to the design aspects of roof trusses before fabrication.

Basic specifications needed when ordering roof trusses for your home or structure include:
# Truss Type
# Truss Span (Length of the bottom chord)
# Number of Trusses
# Truss Bearing Width
# Design Load (Live, wind, dead and other loads to which the trusses are subjected)
# Overhang Length (Space between the bottom edges of the top and bottom chords)
# End Cut (square or plumb cut)
# Heel Height (Space between the top of the top chord and the bottom of the bottom chord)
# Roof Slope, etc

For any queries related to Roof Truss design and related design drawings email us at info@outsourcestructuraldrafting.com

Visit us at http://www.outsourcestructuraldrafting.com/services.php for more information.

About the author: Richard Bose is structural draftsman at OSD – an India based Structural Engineering firm offering full services such as structural steel analysis, structural drafting, shop drawings and structural design services at affordable rates. Email us at info@outsourcestructuraldrafting.com

Frequently Asked Questions

QUESTION:
What are the advantages and disadvantages of traditional and modern pitched roof construction ?

ANSWER:
A steep pitched roof makes water (ice, rain or snow) roll off more quickly. Water is the enemy of the wood under the shingles. You’ll find modern pitched roofs, usually, though not always, in warm climates where the sun & heat evaporates any water problem. However, there is a water barrier that can be applied to modern pitched roofs in northern climates. That is all I know about this topic.

In some instances a high pitched roof with enough of an overhang can create a greenhouse effect, and make your home cooler in the summer.

I suspect one of the advantages of a modern pitch roof (I think you are referring to flat roofs) is they are cheaper to build.

QUESTION:
What are the differences between flat roof construction and pitched roof construction?

ANSWER:
flat roofs are good for region with appreciable wind load whereas pitched roofs are good for region with heavy snowfall and rainfall

in flat roof it is easy to have future expansion which is not possible in case of pitched roof.

in flat roof special care need to be taken for water drainage.

flat roofs are comparatively difficult to construct as compared to pitched roof.

in pitched roof more maintenance is required as compared to other one.

QUESTION:
What are the advantages and disadvantages of both warm and cold pitched roof construction?

ANSWER:
Information is below.

QUESTION:
My house is going under construction (roof) and i want to know a few tips about this?
Security? Can we leave things in the house (clothes, desktop computer etc.) ?
Are they going to knock down the walls ?
Do these people work in the night time or weekends and stuff?
There’s a few areas that need construction can I leave stuff in one room ?

ANSWER:
Don’t you think the people doing the job should be answering these questions for you?

QUESTION:
What is ‘Solid sheet roof and floor construction’?
Looking at buying a shed and it says ‘Solid sheet roof and floor construction’ – is that another way of saying OSB?

ANSWER:
That is another way of saying that you should have a solid service underneath whatever roofing/flooring product you choose to use. Lets take away the solid sheet roof and go with what we call “slats” for metal roofs. You would have your bare rafters ( going vertical), then every two feet i would put wood going across ( horizontal) on top of the rafters. After that point then i would put the metal roof down and mark the sheets of metal to screw them down where ( and only where) those slats are. Because there isnt a solid surface you can only nail/screw in certain areas. So over all its not as strong. Hope this helps you.

P.S. before you buy a pre-fabricated shed. Look at “Angie’s List” ( Google it) for a local framer. Chances are you can find someone to build it for maybe just a little more, but it will last 20 years longer. ( you’ll have to trust me on this one. I would have to explain alot on pre-fab sheds vs ones built like a home)